Efficient temporal compression of coherent nanosecond pulses in acompact SBS generator-amplifier setup

A pulse compressor based on stimulated Brillouin scattering (SBS) in liquids is experimentally and theoretically investigated. It allows for the compression of Fourier-transform limited nanosecond pulses of several hundreds of millijoules of energy with both high conversion efficiency and a high temporal compression factor. The two-cell generator-amplifier arrangement is of a compact design not requiring external attenuation of the generator cell input energy. Pulses from an injection-seeded, frequency-doubled Nd:YAG laser of 300-mJ energy were compressed by a factor variable between 6 and 21 at up to 75% reflectivity. Deviation from unity SBS reflectivity is predominantly determined by optical component losses. The generation of 270-ps pulses with high beam quality was achieved in liquid methanol. These powerful pulses of variable duration are difficult to produce with common laser systems and are highly suited for the generation of high-harmonics in gases     

Experimental study of stimulated Brillouin and Raman scatterings in a Q-switched cladding-pumped fiber laser

Characteristics of stimulated Brillouin and Raman scatterings (SBS and SRS) in a Q-switched cladding-pumped ytterbium (Yb)-doped fiber laser are experimentally investigated. A stable SBS pulse train with a width of ∼2 ns and a peak power of ∼1.0 kW can be generated in regular Q-switching processes under certain conditions. The time jitter and intensity fluctuation of SBS pulses are presented, and their mechanism and statistics are discussed. The accompanied Raman–Stokes waves in regular Q-switched pulses and SBS pulses are quantitatively compared.     

基于SBS超短脉冲激光器的介质研究

皮秒激光器在众多领域中都有广泛的应用,将纳秒量级激光器脉冲压缩至皮秒量级超短脉冲是目前获得高能量皮秒脉冲激光的常见方法之一。而基于布里渊散射(Stimulated Brillouin Scattering)的脉冲压缩系统结构简单,造价较低,是近年来较为热门的技术手段。布里渊散射脉冲压缩方案的技术关键在于介质研究。本文通过研究目前获得应用的三类介质的化学性能、物理特性以及压缩效果等,对比国内外研究现状,分析了受激布里渊散射对介质的要求以及未来的发展方向。     

Fourier-transform-limited laser pulses tunable in wavelength and induration (400-2000 ps)

A combined system of an injection-seeded pulsed dye amplifier and a pulse compressor based on stimulated Brillouin scattering (SBS) is investigated. It allows for the generation of powerful pulses both tunable in wavelength and in duration. Online tuning of the pulse duration is possible due to the dependence of SBS compression on input energy. A range of 400-2000 ps at up to 100 mJ output energy is demonstrated. The output pulses are temporally and spectrally resolved to investigate the properties of this system. Coherent nearly Fourier-transform-limited pulses of variable pulse duration in the extreme ultraviolet (UV) are produced employing harmonic conversion. As an application of such pulses a single rotational line of H₂ at 98-nm wavelength is excited, demonstrating that the system may be used for laser-spectroscopic studies to simultaneously gain spectral as well as dynamical information     

Pulse compression to the subphonon lifetime region by half-cyclegain in transient stimulated Brillouin scattering

A new approach to the transient pulse compression by stimulated Brillouin scattering (SBS) is presented. The theoretical analysis involving the time-dependent SBS gain in explicit form leads to a nonlinear system of partial differential equations, solved numerically by a generalization of the split-step method. It is shown theoretically and confirmed experimentally that the phonon lifetime is not always an appropriate parameter that determines the lower limit to the pulse duration in SBS compressors. A half-cycle gain regime is found for pulses shorter than the phonon lifetime. Hence, under proper conditions, pulses as short as half the acoustic period can be produced     

High-efficiency pulse compression with intracavity Raman amplifiers

High-efficiency pulse compression using intracavity Raman amplifiers has been computed. The energy of a pump laser stored in a lossless cavity is extracted at the Stokes frequency by means of Raman amplification of an input Stokes pulse. Calculations are made for both long and short duration input Stokes pulses for different lossless cavities. As an example, we use a hydrogen-argon mixture as the Raman medium and 1.5 J/cm²energy fluence stored in the lossless cavity at the ruby frequency. By comparing amplified Stokes pulses to a 30 ns pulse duration conventional ruby laser delivering the same energy fluence, pulse shortening factors larger than 20 are computed with quantum conversion efficiencies higher than 80 percent. These values compare favorably to backward Raman amplification. Moreover, this technique is proved to be able to provide a pulse compression rate larger than 14, even for a broad-band laser, which is impossible with backward Raman amplification. This technique could be used with any laser, even with absorbing laser media (excimer lasers) provided pump energy is stored in the lossless cavity by shifting of the laser frequency with any nonlinear process.     

High-energy SBS pulse compression

An efficient two-cell stimulated Brillouin scattering (SBS) pulse-compressor design that can be scaled to large laser-pulse energies is described and a numerical model has been developed that accurately predicts the performance of this pulse-compressor system over a wide range of operating parameters. The compression of a 2.5 J input pulse from a width of 15.8 to 1.7 ns is experimentally demonstrated with 80 percent energy efficiency. A design of an SBS pulse compressor to compress a 25 J pulse to a pulse width less than 1 ns with 80 percent energy efficiency is presented     

Tunable and narrow linewidth multi-wavelength Brill- ouin-erbium fiber laser using dual-wavelength pumping

We demonstrate a multi-wavelength Brillouin-erbium fiber laser (BEFL) with narrow linewidth and tunable wavelength interval using dual-wavelength Brillouin pumping. The generation of multi-wavelength output in BEFL is based on the combination of stimulated Brillouin scattering (SBS) and four-wave mixing (FWM) effect in a fiber cavity. The tunable wavelength interval is determined by the artificially controlled wavelength interval of the pumping lasers. The BEFL could compress a 1 MHz pump laser to a 340 Hz Brillouin Stokes laser, which proves the BEFL has excellent capability of linewidth compression. An erbium-doped fiber pumped by 980 nm laser is inserted into the cavity to further amplify the Brillouin laser. The wideband multi-wavelength BEFL covering over 50 nm is successfully generated when the 980 nm pump power is 400 mW. These features of multi-wavelength BEFL provide an effective method for optical communication systems and optical fiber sensing.     

单频脉冲光纤放大器中的SBS效应实验研究

为了研究受激布里渊散射(SBS)效应对单频脉冲光纤激光放大器中脉冲波形的影响,搭建了单频脉冲光纤激光放大器系统。实验中观测到脉冲波形的畸变,继续提高功率,畸变处出现峰值功率极高的尖峰,分析认为,光纤中的二次受激布里渊散射导致了尖峰脉冲出现。为验证这一结论,构造包含二阶Stokes波的耦合方程组,进行了模拟仿真。结果表明,该峰值功率极高的尖峰正是SBS效应的二阶Stokes波。     

受激布里渊后向散射的模式及时间特性

研究了在各种CS_2介质波导中受激布里渊后向散射的模式特性以及脉宽压缩现象。     

K9玻璃与熔石英玻璃中纵向受激布里渊散射的脉宽压缩和能量提取效率

以K9玻璃与熔石英玻璃代替传统的液体或气体作为纵向受激布里渊散射(LSBS)样品,波长1.064μm,脉宽12 ns的单纵模电光调QNd∶YAG激光器作抽运光,实验上以脉宽压缩和后向斯托克斯(Stokes)散射光频移为标志,测到后向斯托克斯散射光线宽和散射光脉宽压缩比。研究了纵向受激布里渊散射样品的长度、抽运光能量大小、材料种类对纵向受激布里渊散射脉宽压缩效应和散射光能量提取效率的影响。实验结果表明,熔石英与K9玻璃的受激布里渊散射发生阈值、饱和阈值、散射光能量提取效率、散射光脉冲波形均相近,K9玻璃可以成为一种更廉价的受激布里渊散射介质。针对170 mm的熔石英玻璃样品,找到了纵向受激布里渊散射的发生阈值、饱和阈值,并得到了较高的散射光能量提取效率,从100 mm的熔石英玻璃中获得了87%的散射光能量提取效率,而从170 mm的熔石英中获得了90%左右的散射光能量提取效率。实验数据拟合的规律曲线印证了数值模拟结果。     

基于缓变上升沿泵浦光的布里渊慢光研究

为了增加布里渊慢光的时间延迟,减小脉冲展宽,利用迎风格式的有限差分法对光纤中受激布里渊散射耦合方程组进行数值求解,分析了具有缓慢变化上升沿的连续光作为泵浦光时,光纤输出端斯托克斯光的时间延迟和脉冲展宽因子随增益的变化情况。数值结果表明:具有缓慢变化上升沿的泵浦光可使高斯长脉冲FWHM(半宽全高)为120ns的时间延迟增加到52ns,高斯短脉冲(FWHM为20ns)的时间延迟增加到64ns,同时可减小高斯长短脉冲的展宽。     

液态介质纵向振动对受激布里渊散射反射性能的影响

结合静态流体力学理论、弦振动理论及瞬态受激布里渊散射(SBS)理论,建立了描述介质纵向振动所产生的附加密度变化的受激布里渊散射耦合模型。以四氯化碳为介质,研究了不同强度的基频及倍频振动对受激布里渊散射反射率以及波形失真度的影响特征。结果表明,振动对于受激布里渊散射的影响具有一定的阈值性。在低于某一阈值情况下,受激布里渊散射稳定性不受其影响,且受激布里渊散射的振动稳定性在很大程度上受控于受激布里渊散射装置的结构参数。在介质振动强度恒定的情况下,选取短焦透镜及短池长的装置结构更有利于受激布里渊散射相位共轭镜稳定性的提高。     

诱导式受激布里渊散射相位共轭腔

提出利用受激布里渊散射（ＳＢＳ）的相位共轭及脉宽压缩效应,在ＮｄＹＡＧ激光器上构造一种诱导式相位共轭腔装置,实现了能量大于２５０ｍＪ,脉宽小于５ｎｓ的共轭激光输出,并给出了相应的测试结果。     

Gain saturation in fiber Raman amplifiers due to stimulatedBrillouin scattering

The effects of stimulated Brillouin scattering (SBS) on Raman gain are explored. Measurements of gain in a fiber Raman amplifier show a saturation at low gain levels. Experimental data and a theoretical model are presented, demonstrating that this saturation is due to pump depletion by SBS. This effect also leads to Raman gain fluctuations arising from mode partitioning in a multimode pump laser. Two ways to avoid the deleterious effects of SBS on the performance of Raman amplifiers are suggested. One is to use a multimode semiconductor laser with a modal linewidth of the order of a few hundred megahertz, which leads to a strongly reduced Brillouin gain coefficient. The second approach is to use very short pump pulses in a backward configuration     

Effect of laser mode structure on stimulated Brillouin scattering

The gain and reflectivity of the stimulated Brillouin scattering (SBS) process used as a Stokes-wave generator are shown theoretically to be independent of the mode structure of the pump laser provided that the pump-laser mode spacing exceeds the Brillouin linewidth and that the laser coherence length exceeds the characteristic gain length of the SBS process. Under the same set of conditions, the gain of an SBS amplifier is found to depend on the degree of correlation between the laser and Stokes fields. However, due to nonlinear coupling, these two fields become correlated within several characteristic gain lengths, and the subsequent propagation of the two fields is governed by the same set of equations that apply for the case of a single-mode pump laser. These theoretical predictions are tested experimentally for an SBS generator using acetone, carbon disulfide, and methanol as the Brillouin-active media, and the results are in full agreement with the theoretical predictions.     

受激布里渊散射光束空间光强分布的CCD检测

将波长1064nm,脉冲宽度8ns的Nd:YAG激光脉冲聚焦到FC-72介质池中,利用CCD对后向受激布里渊散射(SBS)光斑进行检测,并利用数字图像处理技术对图像进行分析,以获得SBS光斑的空间光强分布和变化规律。利用MATLAB的图像处理工具箱对激光光斑的原始CCD图像进行去噪,并使用Origin软件的曲线拟合和FFT平滑功能对光强分布曲线进行平滑处理。结果表明,进行图像处理后的原图像噪声被有效地抑制和消除,光强分布曲线较为平滑。在入射光束为基模高斯光束的条件下,随着入射光能量增大,受激布里渊散射光斑的空间光强分布由近高斯型转变为高斯型。     

Experimental and modeling studies of a Brillouin amplifier

KrF laser-pumped backward Brillouin amplification of nanosecond pulses at 248 nm is investigated both experimentally and numerically. Gain and saturation of the amplifier system are studied for an SF₆ Brillouin medium at pressures of 5 to 15 atm and 24 ns pump pulses at an intensity of ≈9 MW/cm². The input Stokes intensity is varied from 0.001 to 1.0 MW/cm². Power gains of 20 are achieved at energy extraction efficiencies of 40%. Experimental results are compared to a time dependent numerical model of pulse amplification which incorporates arbitrary pump and Stokes pulse shapes and intensities. The effect of laser bandwidth is investigated in the model calculations in order to assess its influence on Brillouin amplification     

单模光纤中受激布里渊散射对纳秒激光脉冲的光限幅特性

采用 2m长 ,10 0 μm纤径单模光纤 ,研究了受激布里渊散射对纳秒激光脉冲的光限幅特性。从耦合波方程出发 ,利用计算机模拟了光纤中瞬态受激布里渊散射过程的光传输特性。理论模拟的能量及光强度的变化规律表明光纤系统对纳秒激光具有很好的光限幅特性。根据理论分析给出所讨论范围内的实验结果 ,实验结果证明此光学系统对纳秒激光脉冲具有光限幅特性及脉宽压缩特性 ,对于输入能量在 10 0～ 4 0 0 μJ变化的纳秒激光脉冲 ,给出输出能量稳定在 6 5～ 85 μJ范围内 ,与理论分析的结论符合得很好     

Physical mechanism of "slow light" in stimulated Brillouin scattering

We show from analytical analysis of the basic stimulated Brillouin scattering (SBS) equations in the time domain that the SBS amplification process does not amplify an external Stokes pulse and therefore cannot induce group delay of the Stokes pulse as claimed in the literature. Rather, the delayed output Stokes pulse is the pump radiation reflected by the induced acoustic wave, the amplitude of which determines the rate of the amplification process and time delay of the pulse. The latter is predominantly a consequence of the SBS buildup process determined by the inertia of the acoustic wave excitation. Analytical solutions of the SBS equations in the frequency domain show that spectral broadening of the pump radiation leads to only negligible broadening of the SBS spectral bandwidth and so does not provide an effective means to achieve broadband pulse delay.